Hi Pete,
Could well be, although I'd expect it to be a bit later.
German law forced a change of proofing and proof marks. This law was published in june, 1939 and became official from april, 1940 onwards in Germany and from june, 1940 in Germany-controlled areas. Last to join was Austria in july, 1940.
It's very well possible (and most plausible) that Mauser pistols started being proofed 'new style' between june, 1939 and april, 1940.
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